﻿2 W. J. Solkis — On Evolution in Geology. 



temperature of its own, or wliat temperature it has is generally 

 taken as practically constant. 



Thus by raising the temperature of our luminary we increase the 

 difference between its temperature and that of surrounding space. 

 It will consequently cool quicker than when its temperature is lower 

 and the difference in temperature correspondingly less. 



In radiating its heat away more rapidly, it will supply our earth 

 with a greater quantity of energy, which only comes to saying what 

 is obvious enough, that when the sun was hotter, it shone with 

 increased intensity on the earth. 



The Earth. — The same conclusion may be held to be true as 

 regards the earth, likewise considered as a body cooling in space. 

 It is now very hot inside, but at one time it must have been much 

 hotter ; every century that has passed over it, has left it more and 

 more impoverished in heat. It becomes hotter not only as we 

 descend its crust, but also as we pass backwards in time. 



The rate at which it cools can only be determined by making use 

 of data confessedly imperfect, and from these Sir Wm. Thomson 

 finds that 100 or 200 millions of years ago it first began to be 

 crusted over by a solid film of rocks ; that 10,000 j'ears after its 

 first crusting over the temperature of the crust increased 2° F. for 

 every 1ft. vertically descended below the zone of constant tempera- 

 ture. The rate of increase for descent with successive periods is 

 shown in the following table : 



Period after crusting over. Eate of increase in temperatiu-e 

 Years. for every 1ft. descended. 

 10,000 2° F. 



40,000 ... ..-. r 



160,000 



4,000,000 



100,000,000 



Jr of a degree being the present rate of increase. 

 Various estimates have been made by geologists of the time which 

 has elapsed since the deposition of the earliest recorded strata, from 

 the time the Laurentian were first laid down to the present day. 

 From 500 to 50 millions of years would, I think, include the least 

 extravagant of them, a period which brings the period of formation 

 of the earliest known strata into very close proximity with that 

 when the sun first began his existence as our luminary, and when 

 our earth was first permanently crusted over. 



Prof. Thomson's estimates may be wrong, and the estimates of 

 the geologists may be wrong, either or both may be erroneous to a 

 considerable degree ; but even if we put Sir Wm. Thomson's 

 periods farther back, twice as far back, or bring the Laurentian 

 period nearer to us, there will yet remain a very considerable excess 

 of energy, both in the sun and in our planet, beyond what they 

 at present possess, and the influence of which must have been 

 distinctly sensible on the rate of early geologic change. It is there- 

 fore not only legitimate, but it even becomes necessary, to consider 

 what must have been the precise effects of this excess of energy on 

 the earlier physical geography of our globe. 



